In an LTE (Long Term Evolution) being one standard of a radio communication system regulated by 3GPP (3rd Generation Partnership Project), a radio terminal (User Equipment: UE) selects one cell having a downlink (Downlink: DL) component carrier (hereinafter, referred to as a DL carrier) and an uplink (Uplink: UL) component carrier (hereinafter, referred to as a UL carrier) associated with the DL carrier, and performs communication with a radio base station (evolved Node B: eNB) in the cell. Further, a selection of the cell of the radio terminal (UE) basically corresponds to that of the DL carrier. In the LTE, by using the DL carrier, the radio base station (eNB) periodically transmits a reference signal (Cell specific Reference Signal: CRS), a synchronization signal (Synchronization Signal: SS), system information (System Information: SI), and the like that are mutually used by the radio terminals (UEs) within the cell. In the LTE, the above signals and information are signals and information needed when the radio terminal (UE) performs communication with the radio base station (eNB). Further, a transmission configuration (for example, a transmission period or a signal arrangement) is regulated so as to satisfy predetermined conditions.
Next, a handover procedure in the LTE is illustrated with reference to FIG. 15. In addition, in FIG. 15, the radio terminal (UE) is supposed to camp on a Cell 1 and perform communication with the radio base station (eNB) 1.
The radio base station (eNB) 1 performs instruction (MeasConfig) of a terminal measurement report (UE measurement and reporting) to the radio terminal (UE). Here, the instruction (MeasConfig) of the terminal measurement report (UE measurement and reporting) includes information (Measurement objects) indicating targets of the terminal measurement and information (Reporting configurations) indicating a method for reporting terminal measurement results. Further, the above information (Measurement objects) includes information about a radio access technology (Radio Access Technology: RAT), frequencies and cells being targets of measurement, an offset value related to received quality of signals of a measurement target cell used for a determination on report of the terminal measurement results, and the like. Further, the above information (Reporting configurations) includes information indicating whether the report of the terminal measurement results is a periodical report (periodical reporting) or a report (event triggered reporting) by an event trigger, further information indicating which event is performed in the case of the event trigger, and the like. Hereinafter, a case of the event trigger is supposed.
The radio terminal (UE) performs a measurement of the received quality of an indicated cell and a cell of an indicated frequency at the frequency or timing satisfying predetermined requirement, and determines whether the measurement results satisfy conditions of an indicated event. Further, when a situation of satisfying the conditions is continued during a predetermined period (Time To Trigger: TTT), the radio terminal (UE) reports the measurement results (Measurement report).
The radio base station (eNB) 1 determines a handover target based on a report of measurement results received from the radio terminal (UE). When the handover target is a cell (for example, a Cell 3) of a radio base station (eNB) 2, the radio base station (eNB) 1 transmits a handover request (Handover request) to the radio base station (eNB) 2; further, when the handover request can be accepted (namely, the handover of the radio terminal (UE) is permitted), the radio base station (eNB) 2 transmits a positive response for the handover request to the radio base station (eNB) 1 (Handover request acknowledgement). Then, the radio base station (eNB) 1 transmits an instruction of the handover to the radio terminal (UE) (RRC Connection Reconfiguration including MobilityControlnfo) and the radio terminal (UE) performs a handover to the Cell 3 of the radio base station (eNB) 2 based on the above instruction.
By using the above method, an appropriate handover can be realized in consideration of the received quality of signals in a serving cell (Serving cell) and neighbour cells (Neighbouring cell) (Non-patent literature 1).
On the other hand, in 3GPP, there is studied LTE-Advanced, wherein the LTE is enhanced and a function is largely extended. In the LTE, in addition to the reference signal (Cell specific Reference Signal: CRS), the radio base station (eNB) transmits also a reference signal (Demodulation RS: DM-RS) for demodulating user data, a reference signal (CSI-RS) for measuring or calculating a communication path state information (Channel State Information: CSI) of a downlink, a downlink physical control channel (Physical Downlink Control Channel: PDCCH) including scheduling information, and the like. In a study of the LTE-Advanced, it is pointed out that an overhead of the above-described signals or information other than downlink user data is large and usage of downlink radio resources is not optimized; further, for the purpose of improving downlink throughput or cell capacity, the overhead is studied to be reduced. For example, there is studied a method for reducing the number of symbols of the reference signal (CRS) for transmission per sub-frame (Subframe), reducing the number of sub-frames (Subframes) for transmitting the reference signal (CRS), or not absolutely transmitting the reference signal (CRS); further, such types of the DL carriers are called a New Carrier Type (NCT) (Non-patent literature 2). Hereinafter, separately from a conventional component carrier (Legacy Component Carrier: LCC), the component carrier of the NCT is called a New Type Component Carrier (NTCC). The NTCC is supposed to be used as a secondary component carrier (Secondary Component Carrier: SCC) (also called a secondary cell (Secondary Cell: SCell) of a carrier aggregation (Carrier Aggregation: CA) by which the radio terminal (UE) uses a plurality of component carriers at the same time and performs communication with the radio base station (eNB). Here, a component carrier (cell) by which the radio terminal (UE) establishes a connection to the radio base station (eNB) and acquires basic information such as security information is called a primary component carrier (Primary Component Carrier: PCC)/primary cell (Primary Cell: PCell); further, an additional component carrier/cell that is used together with the primary component carrier/primary cell (PCC/PCell) is called the secondary component carrier/secondary cell (SCC/SCell). It can be expected that radio resources for transmitting downlink user data are increased in the secondary component carrier/secondary cell (SCC/SCell) and downlink throughput or cell capacity is improved through an introduction of DL NTCC. Further, power saving of a radio network can be also expected. In addition, the radio base station (eNB) determines the secondary component carrier/secondary cell (SCC/SCell) used by a certain radio terminal (UE) based on the report of the terminal measurement results from the radio terminal (UE). Here, the above-described terminal measurement report for the handover is reused, or that for adding the secondary component carrier/secondary cell (SCC/SCell) is reused.